190 ON MAGNETISM. 



spot of light ; which, if received on photographic paper, 

 may be made to impress a permanent register of the 

 position of the spot, from which the positions of the 

 moving apparatus may be inferred. 



It is now necessary to explain how the time is re- 

 gistered in combination with the register of the spot- 

 movement. For this purpose, the photographic paper 

 must be attached, either to a plane board which is 

 moved by clock-work uniformly in its plane in the 

 direction at right angles to that in which the motions of 

 the spot occur, or to a barrel which is made to rotate 

 uniformly and whose axis is parallel to the motions of 

 the spot. With either of these, the motions of the spot 

 leave on the paper a photographic curve, whose abscissa 

 represents time at a given length for an hour, and whose 

 ordinate represents a quantity proportional to the in- 

 strumental movement which causes the motion of the 

 spot. If we interrupt for a short time the beam of light 

 (which will cause an interruption in the photographic 

 curve), noting also the clock -time, we can mark off 

 accurately the hours, &c., on the time-scale. And if 

 we possess any independent methods of observing the 

 position of the moving apparatus at definite times, we 

 can, by adjusting the scale of ordinates to the spot- 

 position at those times, make it available for every 

 other time. 



The elements which most conveniently represent the 

 state of terrestrial magnetism as acting at anyone geogra- 

 phical point, and whose changes it is desirable to record, 

 ar6j the position of the free magnet, the small changes 



